Microelectrode studies are done in the squirrel monkey and the chinchilla with the purpose of understanding the functioning of the labyrinth in terms of the overall operation of the vestibular system. Four interrelated projects are proposed. I) A combined morphological and physiological approach is used to study the relation between the discharge properties of vestibular-nerve afferents and peripheral innervation patterns in the chinchilla, as well as the cellular mechanisms of repetitive discharge and synaptic transmission within the sensory epithelium. II) An intracellular, electrophysiological paradigm shows that individual secondary neurons in the monkey receive different proportions of their monosynaptic, vestibular-nerve input from regularly and irregularly discharging afferents. To understand the functional significance of these differential projections, they are being correlated with the central connections and locations of secondary neurons determined by a combination of electrophysiological and intracellular dye-injection techniques. III) The head-velocity signal carried by secondary canal-related neurons, which differ from one unit to another, may be related to the distinctive vestibular-nerve inputs, regular or irregular, that each of them receives. The discharge of the secondary neurons are to be recorded in the alert monkey and criteria developed to deduce the profiles of vesticular-nerve inputs by conventional extracellular techniques. IV) Single-unit studies will be done in the alert monkey to investigate possible efferent-induced responses of vestibular-nerve afferents to head and neck rotations. The discharge of brain-stem efferent neurons will be recorded, first in decerebrate monkeys and then, possibly, in the alert animal.